The present invention relates to a method for cooling a plunger tip of a die-casting machine, and more particularly to such method capable of controlling sliding resistance of the plunger tip relative to a shot sleeve.
A die-casting machine has a shot sleeve having a pouring port and a plunger tip slidably disposed in the shot sleeve. The plunger tip is connected to a plunger rod connected to an injection cylinder. The molten metal is poured into the shot sleeve through the pouring port, and the molten metal is introduced into a mold cavity by a reciprocal motion of the plunger tip. In this case, proper sliding resistance is required between an inner peripheral surface of the shot sleeve and an outer peripheral surface of the plunger tip. If large clearance therebetween is provided, the molten metal may be entered into the clearance and solidified thereat, which may cause local galling. This galling at the sliding portion may lower service life of the injection components, and degrade a casted product. Further, if the clearance is too large, the molten metal may be flowed toward a plunger rod, in other words, back flush may occur.
Thus, proper sliding resistance is required for avoiding the galling and the back flush. On the other hand, if excessive sliding resistance is provided, the plunger tip cannot be smoothly reciprocated in the shot sleeve. Accordingly, the clearance between the plunger tip and the shot sleeve must be maintained in an optimum level.
According to one conventional technique, cooling liquid such as water is circulated into the plunger tip having high temperature due to contact with the high temperature molten metal. In an attempt to obtain desired sliding resistance, thermal expansion of the plunger tip is controlled by the control to the cooling liquid supply rate to the plunger tip. However, this control is made by controlling liquid supply valve opening degree on a basis of an operator's skilled judgment on the condition of the die-casting machine.
This control method is fully dependent on the operator's skill and experience. Therefore, the clearance at the sliding portion cannot be properly maintained, if the shot sleeve is made of a special material, such as ceramics and cermet, those having thermal expansion coefficient different from that of the material of the plunger tip, or if the shot sleeve is heated. Consequently, galling or other problems still remain unsolved.
Further, in a recent demand, improvement on maintaining elevated temperature of the molten metal is required. For this, is used water soluble lubricating agent containing solid lubricant. In case of the employment of such lubricating agent, galling may occur due to shortage of fats and oils components and deposition of the solid lubricants in the shot sleeve. Therefore, it would be difficult to maintain proper clearance at the sliding portion.